A MCSDA will allow me to pursue advanced training in molecular genetic and cellular biological approaches to bacterial pathogenesis that would complement and markedly extend my current expertise in veterinary gastroenterology and infectious disease. The proposed training program would help me to develop as an independent, nationally competitive, clinical research scientist, capable of contributing importantly to our understanding of Helicobacter pylori infection and the diseases it causes, and more generally to elucidating mechanisms of pathogen-host interactions that are significant in health and disease, for many years to come. The mentor and the outside collaborator that I have chosen are internationally recognized in bacterial pathogenesis and genetics, and molecular cell biology, and have strong records as research mentors. The studies proposed herein aim to clarify the relative importance of host and bacterial factors in determining the clinical consequences of infection with H. pylori. I will test and extending the hypothesis that "the ability of a given H. pylori strain to colonize, invade and damage the gastric mucosa of a given host, and the response that it induces depend strongly on its genotype." The proposed studies utilize a cat-model of H. pylori infection. Cats can become naturally colonized with H. pylori within a few months of birth, have a gastric milieu similar to humans, and develop a mucosal inflammatory response that is more comparable to H. pylori infection in people (e.g. IL-8 upregulation) than widely used rodent models. Aim 1 will identify H. pylori strains of recent human origin that can colonize and induce histopathological changes in cats. Aim 2 will generate derivatives of virulent H. pylori strains of interest with knockout mutations in key virulence-associated genes, and in particular test the importance in colonization, of the remarkable association of toxigenic alleles of the vacuolating cytotoxin gene (vacA), the cytotoxin associated antigen gene (cagA), and other genes of the cag pathogenicity island (PAI). The related Aim 3 will use those sets of isogenic mutant strains to examine the roles of vacA, cagA and other PAI genes in inducing, or down regulating, pathologies such as gastric mucosal inflammatory and systemic immune responses, hypergastrinemia, and attachment, invasion and damage of mucosal cells. Studies in cats will be correlated with cell and organ culture-based analyses, including induction of IL-8 secretion and the ability to damage, invade and efface. I expect that these studies will provide important insight into bacterial factors that influence H. pylori colonization and pathogenicity. The training proposed herein would allow me to fully develop the skills needed to study bacterial pathogenesis at the cell and molecular levels, and would enable me to pursue a career as a clinical scientist with unique qualifications to explore pathogenic mechanisms at the interface between basic cell biology and clinical practice.